Inkjet printers operate by ejecting small droplets of ink from individual orifices in an array of such orifices provided on a nozzle plate of a printhead. The printhead may form part of a print cartridge which can be moved relative to a sheet of paper and the timed ejection of droplets from particular orifices as the printhead and paper are relatively moved enables characters, images and other graphical material to be printed on the paper.
A typical conventional printhead is fabricated from a silicon substrate having thin film resistors and associated circuitry deposited on its front surface. The resistors are arranged in an array relative to one or more ink supply slots in the substrate, and a barrier material is formed on the substrate around the resistors to isolate each resistor inside a thermal ejection chamber. The barrier material is shaped both to form the thermal ejection chambers, and to provide fluid communication between the chambers and the ink supply slot. In this way, the thermal ejection chambers are filled by capillary action with ink from the ink supply slot, which itself is supplied with ink from an ink reservoir in the print cartridge of which the printhead forms part.
The composite assembly described above is typically capped by a metallic nozzle plate having an array of drilled orifices which correspond to and overlie the ejection chambers. The printhead is thus sealed by the nozzle plate, but permits ink flow from the print cartridge via the orifices in the nozzle plate.
The printhead operates under the control of printer control circuitry which is configured to energise individual resistors according to the desired pattern to be printed. When a resistor is energised it quickly heats up and superheats a small amount of the adjacent ink in the thermal ejection chamber. The superheated volume of ink expands due to explosive evaporation and this causes a droplet of ink above the expanding superheated ink to be ejected from the chamber via the associated orifice in the nozzle plate.
Many variations on this basic construction will be well known to the skilled person. For example, a number of arrays of orifices and chambers may be provided on a given printhead, each array being in communication with a different coloured ink reservoir. The configurations of the ink supply slots, printed circuitry, barrier material and nozzle plate are open to many variations, as are the materials from which they are made and the manner of their manufacture.
The typical printhead described above is normally manufactured simultaneously with many similar such printheads on a large area silicon wafer which is only divided up into individual printhead dies at a late stage in the manufacture. FIG. 1 is a plan view of the front surface of a substantially circular silicon wafer 10 typically used in the manufacture of printheads. The wafer 10 has a large number of slots 12 each extending fully through the thickness of the wafer. In FIG. 1 the slots 12 are grouped in threes, as would be the case where the wafer is to be used in the manufacture of printheads for colour printing. The rear surface (not seen in FIG. 1) of the wafer 10 has grooves running vertically between each group of three slots 12 and horizontally between each row of slots 12 so that ultimately the wafer can be divided up, for example, using a conventional dicing saw into individual “dies” each containing one group of three slots 12.
In the final printhead each slot 12 supplies ink to one or more ink ejection chambers disposed along one or both sides of the slot on the front surface of the wafer. Although, for reasons of mass production, the ink supply slots 12 are almost always formed in the undivided wafer 10, they can be formed at any of a number of different stages of production. However, although the slots 10 can be formed in the initial “raw” wafer, as seen in FIG. 1, it is preferred to form the slots when the front surface of the wafer already bears the thin film resistors and other circuitry. This is because an unslotted wafer presents an uninterrupted front surface for the application and patterning of the various layers forming the thin film circuitry. If the slots were present they would need to be temporarily blocked off, for example, in the manner disclosed in European Patent Application No. EP 1,297,959, or other measures would need to be taken to avoid leaving undesired materials in the slots.
However, if the slots are formed when the front surface of the wafer already bears the thin film circuitry, the latter needs to be covered with a protective coating to avoid damage to the delicate and critical thin film structures. A coating of polyvinyl alcohol (PVA) is conventionally used to protect these structures. For example, a typical protective coating is built up by applying five successive layers of PVA each approximately 2.5 microns thick.
The slots 12 are conventionally formed by laser machining or sand blasting, usually from the rear surface of the wafer. Laser machining is preferred since sand blasting leads to dimensional instability and chipping. However, we have found that conventional PVA coatings provide acceptable protection for the critical thin film structures only when slotting with relatively low power lasers, e.g. 7.5 W lasers, and then only when slotting from the rear surface of the laser. The reason is that the high plasma temperature associated with higher power lasers, such as 15 W and 20 W lasers, tends to lift the PVA coating at the edges of the slot when breaking through the front surface (whether from the front or rear), so that the laser machining plasma gets under the edges of the PVA to damage the thin film circuitry and deposit wafer debris thereon. Quite apart from the desirability of reducing the damage caused by higher power lasers, it would be desirable to be able to effect slotting from the front surface of the wafer since then the wafer can be slotted simultaneously from both the front and rear surfaces to improve throughput.
It is an object of the invention to provide an improved method of making an inkjet printhead in which these disadvantages are avoided or mitigated.